1. Field of the Invention
The present invention relates to a method of manufacturing a magnetic recording medium, in particular to such a method that provides a bit patterned media (BPM) with high precision having multiple magnetic recording elements that are separately distributed. The invention also relates to a magnetic recording medium manufactured by the method.
2. Description of the Related Art
Magnetic disk drives are expanding their field of application and are tending to decrease in size. Magnetic recording media mounted on the magnetic disk drives thus are required to continually enhance their recording density. With the recent demand for high density recording of the magnetic recording medium, the magnetic recording scheme is changing from the conventional longitudinal recording scheme to the perpendicular recording scheme, the latter being capable of higher recording density.
With reduction in bit size for the purpose of raising the recording density in this situation, a problem has become significant that the recorded signal degrades due to easier random inversion of magnetization caused by thermal fluctuation of the magnetic substance. In order to hinder the thermal fluctuation to suppress this degradation of signals, it is possible to use a material with a large crystalline magnetic anisotropy energy constant for a magnetic recording layer. A material with a large value of this constant, however, exhibits a large coercivity, with the consequence of a large magnetic field required for magnetization inversion, which raises another problem of difficulty in the information write process.
Therefore, a multiple of proposals have been made for the thermally assisted recording technology to avoid degradation of recorded signals caused by the thermal fluctuation of the magnetic substance. The following are some examples of disclosures regarding this technology.
Japanese Unexamined Patent Application Publication No. 2006-012249 discloses a thermally assisted magnetic recording device comprising a substrate and a hard magnetic recording layer formed on the substrate, wherein the exchange coupling energy between magnetic grains composing the magnetic layer is large at room temperature to thermally stabilize the magnetization of the magnetic grains, and the exchange coupling energy is decreased during the recording process by heating the recording layer with a heating means to sharpen the gradient of recording magnetization transition.
Japanese Unexamined Patent Application Publication No. 2003-045004 discloses an optically assisted magnetic head comprising a floating slider that flies owing to rotation of a disk having a recording medium, an optical waveguide that is provided on the floating slider and emits laser light of a semiconductor laser from an emission end, a near field light minimizing means that minimizes a near field light formed at the emission end of the optical waveguide by the laser light from the semiconductor laser and emits near field light onto the recording medium to heat the recording medium, and a thin film magnetic transducer that is provided on the floating slider and records information on the area of the recording medium heated by the near field light minimizing means.
FUJITSU, Vol. 58, No. 1, p. 85-89 (2007), a Japanese language article, discloses thermally assisted magnetic recording that utilizes Curie temperature recording. In the document, the possibility of thermally assisted magnetic recording has been studied by assembling a write-read tester and using a conventional optical head, a recording medium, and a magnetic head. The document describes that write-read is capable on a high coercivity medium of Hc=13 kOe exhibiting good thermal fluctuation resistance and the recording width is narrowed down to 30% or less as compared with a magnetic disk at that time (100 Gbit/in2).
In the thermally assisted recording technologies disclosed in Japanese Unexamined Patent Application Publication No. 2006-012249, Japanese Unexamined Patent Application Publication No. 2003-045004, and FUJITSU, Vol. 58, No. 1, p. 85-89 (2007), the recording medium is heated to a temperature near the Curie temperature by laser light irradiation or the like to decrease the coercivity and recording is carried out at this condition. After that, the recording medium is quenched to increase the coercivity and information is stored in this condition. This technology allows writing at a low magnetic field and ensures long term stability of a magnetic substance without the influence of thermal fluctuation.
However, when the above-mentioned thermally assisted recording technology is used in a conventional discrete track medium in which magnetic recording elements are continuously arranged, a problem exists that adjacent magnetic recording elements are also heated raising their temperature.
Additional problems accompany the desired increase in recording density of the magnetic recording medium, including a problem of cross talk, that is, a write on the adjacent track other than the intended track due to divergence of the recording magnetic field of the magnetic head and a problem of cross read, that is, a read of a signal in a track other than the intended track.
These problems can be eliminated if the magnetic recording elements are isolated thermally and magnetically at the same time. Thus, a technology called a bit patterned medium (BPM) technology has been proposed for the purpose of thermally and magnetically isolating magnetic recording elements from one another.
Some specific techniques of the BPM technology have been proposed to date. A technique among them utilizes alumina nano holes (ANHs) formed by anodizing aluminum.
The ANHs are formed in self organization in the process of anodization originating from micro pits formed on an aluminum-containing layer. Consequently, when the micro pits are arranged in a triangular lattice pattern or a square lattice pattern, the obtained ANHs are also arranged in the triangular lattice pattern or in the square lattice pattern. Some techniques for the ANHs have been disclosed as shown in the following.
FUJITSU, Vol. 58, No. 1, p. 90-98 (2007), a Japanese language article, discloses anodized porous alumina formed by anodizing aluminum. The document describes the anodized porous alumina concerning control of the nano hole configuration and application to a mold process, and states that the nano hole configuration can be controlled in a nanometer scale and consequently, the anodized porous alumina is suited for a starting material in forming various shapes of nano structures. The document further says that the anodized porous alumina has various merits including good thermal resistance and high mechanical strength, and the possibility to provide a membrane with the nano holes perpendicular to the film surface, and is thus applicable to processes for composing a variety of materials.
Japanese Examined Patent Application Publication No. S51-021562 and Hideki MASUDA et al., HYOUMEN KAGAKU (Surface Science), Vol. 25, No. 5, p. 260-264 (2004), a Japanese language article, disclose nano hole patterned media comprising nano holes obtained by anodizing aluminum. Hideki MASUDA et al. set forth development of a process to obtain a disk of a nano hole patterned medium and succeeded in evaluating the write-read characteristics of a magnetic head, elucidating an effect of minimizing the nano holes. The document also states that a performance improvement effect has been shown by providing a soft magnetic underlayer film enabling write-read with a magnetic head for perpendicular magnetic recording. The document further describes the development of a one dimensional arrangement technique to align the nano holes in the circumferential direction and the prospect for achieving a recording of 1 T bit/in2.
In the technology disclosed in Japanese Examined Patent Application Publication No. S51-021562 and in Hideki MASUDA et al., HYOUMEN KAGAKU (Surface Science), Vol. 25, No. 5, p. 260-264 (2004), the alumina in which the ANHs are formed is a non-magnetic material and has a very small thermal conductivity. Utilizing these properties of the alumina, studies have been made to form a magnetic recording medium in which the magnetic recording elements are thermally and magnetically isolated and one hole corresponds to one bit, by filling the arrangement-controlled ANHs with a magnetic substance by means of an electroplating method.
In addition to the Japanese Examined Patent Application Publication No. S51-021562 and the above-mentioned Hideki MASUDA et al. article, Japanese Unexamined Patent Application Publication No. 2005-120421, for example, discloses a magnetic recording medium using a multiple of nano holes (ANHs) formed by anodizing an aluminum-hafnium alloy layer.
Japanese Unexamined Patent Application Publication No. 2003-321798 discloses an electro-chemical treatment method, such as electroplating, in which electrode materials for a cathode and an anode are installed in a reactor vessel containing electrolytic substances and a sub- and super-critical state is established in the reactor vessel containing the electrolytic materials and electrolytic solution. Under such a state, the electrode material is electrolyzed or the electrolyzed electrode material and/or the electrolytic materials contained in the electrolytic solution are deposited and adhere to the other electrode material. (See claim 1 of this document).
Paragraph 0059 of the above Japanese Unexamined Patent Application Publication No. 2003-321798 sets out that, after discharging the carbon dioxide being used, valve 38 is closed and valve 25 is opened to introduce the high pressure carbon dioxide 7 into the plating bath 1, pressurizing and heating the inside of the plating bath 1, thereby establishing a super-critical state of the carbon dioxide, which comes in contact with the article 4 to be treated. Thus, the moisture component adhered on the article 4 and the anode 3 is eliminated and dried rapidly and efficiently.
The above Japanese Unexamined Patent Application Publication No. 2003-321798 further states that the technique to deposit and stick one electrode material onto the other electrode material can be applied to processes of electroforming and anodized-film forming, which are similar in principle to the invented technique of Japanese Unexamined Patent Application Publication No. 2003-321798 (see Paragraph 0069 therein).
Even in the magnetic recording media in the BPM technology using ANHs exhibiting good characteristics as described above, the following problem has become apparent with the progress of the efforts for enhancing the recording density.
Specifically, an amount of filled magnetic substance is apt to be uneven in filling fine ANHs with the substance by means of an electroplating method. The phenomenon is significant for ANHs with a diameter not larger than 25 nm, the reason for which is described in the following.
The relation among a radius, r, of an ANH, a surface tension, γ, of a plating liquid with the ANH, and a pressure difference, ΔP, necessary for the plating liquid to enter the ANH is given by the Formula (I) called Laplace's equation as follows.ΔP=2γ/r  (1)
According to Formula (I), with decrease in the ANH radius, r, the pressure difference, ΔP, necessary for the plating liquid to enter the ANH increases in inverse proportionality to the ANH radius, r. As a result, the plating liquid hardly enters the ANH with a diameter not larger than 25 nm, thus failing to uniformly fill the ANHs with a magnetic substance by a plating method.
Although Japanese Unexamined Patent Application Publication No. 2003-321798 discloses rapid and efficient elimination and drying of the moisture component adhered on the treated article using super-critical carbon dioxide, the document does not mention or suggest the above-described problem that the plating liquid hardly enters the ANH with a diameter not larger than 25 nm, thus failing to uniformly fill the ANHs with magnetic substance by the plating method.
It is therefore an object of the present invention to provide a method of manufacturing a magnetic recording medium using ANHs in which the ANHs are filled with a magnetic metal uniformly and selectively even for ANHs with a diameter not larger than 25 nm.